This invention relates to a process for making block copolymers of vinyl aromatic hydrocarbons and/or conjugated dienes and polydimethylsiloxane.
Linear block copolymers of polystyrene and polydimethylsiloxane have been synthesized, both by graft and block copolymerization. In block copolymerization of such linear polymers, polystyrene is produced by anionic polymerization with an organo lithium promoter in a solvent and the living polymer (PS.sup.- Li.sup.+) created thereby is reacted with hexamethylcyclotrisiloxane, (Me.sub.2 SiO).sub.3, in the presence of a polar promoter, also in a solvent, wherein a block of polydimethyl-siloxane grows on the end of the living vinyl aromatic hydrocarbon polymer block.
Such polymers incorporate the strength of vinyl aromatic hydrocarbons such as polystyrene and the high flexibility of polydimethylsiloxane and its extremely low solubility parameter compared with polystyrene. These polymers are useful for surface-inactive coatings and in impact modification of engineering thermoplastics. The present invention produces linear block copolymers of vinyl aromatic hydrocarbons and polydimethylsiloxane which are highly phase separated and have a very low Tg for the rubber black.
The manufacture of block copolymers of polystyrene and polydimethylsiloxane has, to date, necessarily involved two separate polymerization steps. First, polystyrene-is anionically polymerized to the desired molecular weight and only then can the hexamethylcyclotrisiloxane be added to polymerize the polydimethylsiloxane block on the polystyrene polymer. This multiple polymerization method has been used extensively for block copolymers of vinyl aromatic hydrocarbons and conjugated dienes because polymerizing such polymers in the presence of both monomers at the same time leads to tapering between the blocks which causes poor phase separation and broad molecular weight distribution in the final polymer, thereby affecting properties negatively, e.g., low strength. It would be advantageous to be able to produce block copolymers of vinyl aromatic hydrocarbons and polydimethylsiloxane in the presence of both monomers if the polymer produced had no tapering between the blocks and an acceptably narrow molecular weight distribution.